Optical storage media are media in which data is stored in an optically readable manner, so that they can be read, for example, by means of a laser and a photodetector being integrated in a pickup. Current generation commercial optical storage media include single- and dual-layer DVD and Blu-ray discs, in which recording and playback are based on controlling or detecting returned light from reflective layers within the medium (i.e. an optical disc). Of these, the highest data storage capacity is achieved by a dual-layer Blu-ray disc, which can contain up to about 50 gigabytes of information. However, emerging applications, for example in very large data centres, will require even higher density storage in future, in order to minimise the physical space required for data storage, as well as the energy requirements for maintaining and operating such data centres.
One technique for increasing storage density is to utilise all three dimensions of the storage medium, i.e. by storing additional data at different depths within the medium. Dual-layer DVD and Blu-ray discs are examples of this technique, and allow independent storage of data in two discrete recording layers, which are laminated within the disc structure, and accessed by adjustment of the laser beam focus. The number of discrete layers that can be incorporated into the recording medium in this manner is limited by physical characteristics, e.g. weight and thickness, and the data density in the depth dimension is also limited by the physical spacing of the layers.
A second technique for increasing storage density is to improve the optical resolution of the storage system. Conventionally, the resolution of optical data storage is limited by the diffractive nature of light. Higher density can be achieved either by increasing the numerical aperture of the optics, or reducing the wavelength (i.e. increasing the frequency) of the optical sources used for writing/reading. In any event, however, it is difficult to form a recording feature size smaller than a half wavelength of the writing beam, or conversely to detect a feature smaller than the half wavelength of the reading beam.
Recently, far-field super resolution recording methods have been developed which employ special polarisation states of the writing beam, or in which the pupil function at the back aperture of the objective is apodised. However, these methods cannot achieve a resolution below 50 nanometers. Additionally, bit sequential recording using these methods is intrinsically slow, and accordingly the data throughput is limited.
The most desirable characteristics for ultra-high density optical data storage are therefore high optical resolution, the ability to utilise all three dimensions of a bulk recording medium (i.e. without the need to laminate separate, discrete recording layers from differing materials), and high data throughput, both for recording and reproduction. There is an ongoing need for improved optical data storage methods and systems which are able to achieve superior performance across these key criteria.